Liquid, its use for the preparation of powder mixtures on the basis of iron or stainless steel as well as a method for the preparation of powder mixtures on the basis of iron or stainless steel

ABSTRACT

A liquid for the preparation of powder mixtures on the basis of iron or stainless steel, comprising water, an inhibitor, and a stabilizer, and a method of forming the same.

The invention relates to a liquid, its use for the preparation of powder mixtures of iron or stainless steel as well as a method for preparation of powder mixtures on the basis of iron or stainless steel.

In powder metallurgy, metal based powder mixtures in particular on the basis of iron or stainless steel are used in order to manufacture herefrom highly stress resistant parts.

For this purpose suitable iron or stainless steel powder mixtures are at first prepared, pressed to form a molded part and the molded part produced thereby (the green body) is then sintered to the finished part.

Powder mixtures on the basis of iron or stainless steel are generally compounded from an iron powder, an alloy powder, such as for instance copper, graphite or iron phosphite powder as well as another material, which is effective as an auxiliary pressing aid.

For the auxiliary pressing aid, effective materials are usually waxes or stearates, for example zinc, aluminum, lead or especially ethylene-bis-stearate (EBS) are employed.

Powder mixtures of the type mentioned above tend to separate out again. At the end of the preparation process for example of the mixing step an optimal homogeneous powder mixture is generally not present. The molded bodies manufactured from the correspondingly inhomogeneous powder mixtures frequently have inadequate properties.

It is moreover problematic that the powder mixtures which are prepared as above, just lack green strength. The form shaping and subsequent conservation of the shape of the green body is thus very complicated. Finally the lacking green strength can lead to the sintered molded body having inadequate properties.

Finally, the powder mixtures as prepared above tend to form “dust” since the fines are not bonded sufficiently into the powder mixture.

The basic object of the invention is to demonstrate a way of preparing powder mixtures on the basis of iron and/or stainless steel, while avoiding the disadvantages described above.

This object is solved in accordance with the invention by providing a liquid for the preparation of powder mixtures on the basis of iron or stainless steel which contains

-   -   water     -   inhibitor     -   stabilizer and     -   further materials if necessary

The basic recognition of the invention is that the disadvantages in the preparation of powder mixtures on the basis of iron and/or stainless steel described above can be prevented, if the powder mixtures are not—as hitherto—prepared dry, but are prepared in the liquid in accordance with the application, i.e. especially are mixed herein.

For that the powder mixture on the basis of iron or stainless steel is initially mixed in the liquid in accordance with the application and subsequently dried.

The powder mixture can alone comprise an iron and/or stainless steel powder as well as an alloy powder; the addition of an auxiliary pressing aid such as a powdered stearate can be dispensed with.

In order to prevent reaction of the water with the iron or stainless steel powder, then especially for example to prevent its hydrolysis the liquid has at least one inhibitor in addition to the water.

Inhibitors have the object of coating the iron or stainless steel based powder mixture or controlling the accumulation of oxygen onto the particles of the powder mixture in such a way that they do not hydrolyze in the water or undergo any other reaction with the water.

Earlier investigations of mixing powder mixtures based on iron in water regularly failed in that a reaction of the iron particles with the water could not be prevented. By means of an inhibitor, especially through one or a plurality of the inhibitors named below, this reaction can henceforth be completely or at least extensively forestalled.

As inhibitors, for example benzoates, ethanolamine boric acid, phosphates and phosphonates as well as benzotriazole, tolytriazole, sodium mercaptobenztriazole, aminoalkylbenzimidazol and mixtures thereof can be used. Alternatively or cumulatively molybdates, nitrites, sebacates, silicates and mixtures thereof can be employed as inhibitor.

Up to now it was unknown that these substances are effective as excellent inhibitors in the liquid preparation of powder mixtures on the basis of iron or stainless steel or are effective in the mixing of iron and stainless steel based powder mixtures in water.

As other materials the liquid in accordance with the application can contain auxiliary pressing aids.

As auxiliary pressing aids, wax emulsions can for example be employed. The wax emulsion can for example be a wax emulsion on the basis of paraffin, oxidized paraffin, polyethylene, polypropylene, stearates, amide waxes such as for example ethylene-bis-stearylamide, stearylamide or mixtures thereof.

It was recognized in accordance with the invention that wax or a stearate is effective as a pressing aid and also in addition as an inhibitor in a water-based liquid in which powder mixtures on the basis of iron or stainless steel are mixed. The wax or stearate can however only then be employed when it is present in the liquid as an emulsion. The addition of wax or a stearate in non-emulsified form would not lead to any distribution of wax in the water. Correspondingly earlier tests failed to use water as liquid for the preparation of iron-based powder mixtures since the non-emulsified wax or stearate does not separate or distribute in the water.

The viscosity of the liquid is controlled by means of the Stabilizer. In particular stabilizers in accordance with the application are employed which increase the viscosity of the water so that the settling of the components of the powder mixture in the liquid is made difficult and demixing is thus opposed.

As stabilzers, for example celluloses can be employed, for example celluloseether, for example hydroxyethylcelluloses, hydroxypropylcelluloses, methylcelluloses, methylethylhyroxycelluloses, methylpropylhydroxycelluloses or mixtures thereof. Alternative or cumulatively alginates, starches, starchethers, guar, guarether, xanthane or mixtures thereof can be employed as stabilizers.

Cumulatively or alternatively to the inhibitors named above, the liquid in accordance with the application can also contain as inhibitor especially carboxylic acids and/or amines or their particular derivatives. Especially carboxylic acid derivatives in the amine neutralized state prove to be particularly effective inhibitors.

As carboxylic acids, preferably dicarboxylic acids and/or triazine derivatives are employed. Dicarboxylic acids can for example be especially 1,8-octanedicarboxylic acid, 1,9-nonane dicarboxylic acid, 1,10-decanedicarboxylic acid (sebacic acid), 1,11-undecanedicarboxylic acid, 1,12-dodecanecarboxylic acid and so forth up to including 1,16-hexadecanedicarboxylic acid. Relative to the triazine derivatives for example especially 1,3,5-triazine derivatives, above all 1,3,5-triazine-2,4,6-triaminetricarboxylic acid are provided.

As amines, preferably ethanolamine and/or imidazolines are employed. Ethanolamines can for example in particular be triethanolamine, diethanolamine, monoethanolamine and/or trishydroxymethylaminomethane. Relative to the imidazolines, in addition to polymers containing imidazole units also non-polymeric imidazole derivatives, like for example 2-(2-heptadec-8-enyl-2-imidazoline-1-yl)ethanol are provided. Also similar imdidazo compounds to the latter can be provided, for example imidazoline derivatives, such as for example hydroxyethyl-, aminoethyl- and/or amidoethyl-imdidazol derivatives. As alkyl groups oleic and talloleic fatty acids up to octanoic acid can be present.

It was determined in accordance with the invention that the above named inhibitors are particularly effective when they are present in combination with amines and carboxylic acids, in each case especially of the previously stated type.

Surprisingly it was further determined, in accordance with the invention that without exception the inhibitors named above, especially also the previously mentioned carboxylic acids and amines can control the viscosity of the liquid, they can thus function as stabilizers. According to one embodiment it is therefore provided to waive the addition of a separate stabilizer. In this case the inhibitors used represent at the same time stabilizers within the intent of the application.

At least one of the other materials in the liquid can for example be a dispersion agent, for example polyacrylic acids.

A temporary binder can also be present in the liquid as another material, for example a temporary binder on the basis of polyacrylates, polyvinylacetates, polyvinylalcohols as well also as cellulose and starch derivatives or mixtures thereof.

The preparation of powder mixtures on the basis of iron or stainless steel are not only strongly improved through the use of the liquid in accordance with the application, but also the properties of the sintered molded parts produced from these mixtures are markedly improved compared to the molded parts according to the prior art.

So by use of the liquid in accordance with the application powder mixture dust can be almost completely prevented.

Also the green strength of the formed green body can be significantly increased.

The (sintered) molded parts manufactured from the powder mixtures which have been prepared with the liquid in accordance with the application exhibit excellent properties especially high homogeneity and strength.

For the preparation of an iron and/or stainless steel based powder mixture with the liquid in accordance with the application, the powder mixture is initially mixed in the liquid in accordance with the application and subsequently dried.

The homogenization of the powder mixture in the liquid in accordance with the application can be carried out in known mixing units, for example attritors.

The mixing duration in the liquid can amount to 1 to 3 hours.

After the mixing of the powder mixture in the liquid in accordance with the application the moist powder mixture is taken to a drying plant for example a spray drying installation.

In the drying plant, for example a spray drying installation, the moist mixture is atomized to produce a granulate.

Thereby a characteristic granulate forms from the mixed powder mixture mixed in the liquid in accordance with the application, which in addition to whole granular portions is also characterized by hollow granulate.

On the basis of the regularly formed proportion of hollow granulate the dried granulate can be very effectively formed by means of forming tools, customarily in presses, into green bodies.

Damage to the forming tool especially of the moving parts of suchlike in the event of granulate dust is prevented by means of the use of the liquid in accordance with the application.

As already indicated above, it was determined in accordance with the invention that the wax emulsion in addition to its property as a pressing aid, acts at the same time as an excellent inhibitor, whereby the homogeneity and green density or strength of the pressed molded part is increased at the same contact pressure (in comparison to the contact pressure or the properties of a molded part, that was prepared according to the prior art) or a lower contact pressure is required in order to obtain the same properties for the molded part, which was obtained without a wax emulsion in the liquid in accordance with the application.

The green body is finally subjected to heat treatment and thereby sintered to the iron or stainless steel product.

The liquid in accordance with the application can be compounded for example as follows (the following data in wt % are, unless stated otherwise, with respect to the total weight of the liquid):

-   -   water: 25-99 wt % thus for example thus 68-98 wt % or 82-93 wt         %;     -   inhibitor: 0.02-50 wt %, thus for example also 1-7 wt % or 1-4         wt %;     -   stabilizer: 0.005-7 wt %, thus for example also 0.05-4 or 0.1-1         wt %;     -   other materials: 0-20 wt %.

Wax emulsion can for example be present in the liquid in proportions from 0.5-35 wt % thus for example also in proportions from 1-15 wt % or 4-12 wt %. Carboxylic acid can for example be present in the liquid in proportions from 0.1-5 wt % thus for example also in proportions from 0.5-4 wt %.

Amines can for example be present in the liquid in proportions from 0.1-7 wt % thus for example also in proportions from 0.5-5 wt %.

Dispersion agent can for example be present in the liquid in proportions from 0.01-33 wt % thus for example also in proportions from 0.05-6 wt % or from 0.1-3 wt %.

The liquid in accordance with the application can for example be mixed with the moist mixture at 3 to 7 times the amount of an iron or stainless steel based powder mixture.

In the following, three examples are given for a liquid in accordance with the application:

EXAMPLE 1

-   -   water: 88.2 wt %;     -   inhibitor in the form of monoethanolamine borate: 2.2 wt %;     -   stabilizer in the form of methylcellulose: 0.4 wt %;     -   wax emulsion in the form of a 50% paraffin emulsion: 8.8 wt %;     -   dispersion agent in the form of polyammoniumacrylate: 0.4 wt %.

EXAMPLE 2

-   -   water: 84.7 wt %;     -   inhibitor in the form of monoethanolamine phosphate: 2.8 wt %;     -   stabilizer in the form of methylhydroxypropylcellulose: 0.6 wt         %;     -   wax emulsion in the form of a 50% polypropylene emulsion: 11.3         wt %;     -   dispersion agent in the form of polyammoniumacrylate: 0.6 wt %.

EXAMPLE 3

-   -   water: 88.2 wt %;     -   inhibitor in the form of 1,10-decanedicarboxylic acid: 2.1 wt %;     -   inhibitor in the form of triethanolamine: 2.1 wt %     -   wax emulsion in the form of a 50% paraffin emulsion: 7.6 wt %.

In example 3 the inhibitors at the same time perform the function as stabilizer.

In the moist powder mixture of the liquid in accordance with the application and the iron/stainless steel powder mixture, besdides 100 wt % iron/stainless steel particles, for example the following proportions of the liquid components can additionally be present:

-   -   8-200 wt % water, thus for example also 12-70 or 12-30 wt %         water;     -   0.05-10 wt % inhibitor, thus for example also 0.2-2 wt %         inhibitor;     -   0.01-5 wt % stabilizer, thus for example also 0.05-1 or 0.05-0.5         wt % stabilizer.

Besides the 100 wt % iron/stainless steel particles and the components previously mentioned for example 1-5 wt % wax emulsion, thus for example also 0.1-20 or 0.5-4 wt % wax emulsion and/or 0.01-5 wt % dispersion agent thus for example also 0.05-1 dispersion agent can be present in the moist mixture. 

1. A liquid for the preparation of powder mixtures on the basis of iron or stainless steel, containing a) water, b) inhibitor, c) stabilizer and d) further materials as necessary.
 2. The liquid according to claim 1, in which the inhibitor is at least one of the following materials: benzoates, ethanolic boric acid, phosphates and phophonates as well as benzotriazole, tolytriazole, sodium mercapto benzotriazole, amino alkyl benzimidazoles, molybdates, sodium nitrite, sebacates or silicates.
 3. The liquid according to claim 1, in which the inhibitor is at least one of the following materials: carboxylic acids, amines or their derivatives.
 4. The liquid according to claim 1, in which the stabilizer is at least one of the following materials: cellulose ether, alginate, starch, starch ether, guar, guar ether or xanthane.
 5. The liquid according to claim 1, in which at least one of the further materials is a wax emulsion.
 6. The liquid according to claim 5 with a wax emulsion on the basis of at least one of the following materials: paraffin, oxidized paraffin, polyethylene, polypropylene or amide wax such as for example ethylene-bis-stearylamide, stearylamide or stearate.
 7. The liquid according to claim 1, in which at least one of the other materials is a dispersion agent.
 8. The liquid according to claim 7, in which the dispersion agent is a polyacrylic acid.
 9. The liquid according to claim 1, in which at least one of the other materials is a temporary binder.
 10. A use of a liquid according to claim 1 for the preparation of powder mixtures on the basis of iron or stainless steel.
 11. The use of a liquid according to claim 10 as mixing liquid for the preparation of powder mixtures on the basis of iron or stainless steel.
 12. A method for the preparation of powder mixtures on the basis of iron or stainless steel, in which a powder mixture on the basis of iron or stainless steel is initially mixed in a liquid in accordance with claim 1 and is subsequently dried
 13. The method according to claim 12 in which the drying is carried out in a spray drying unit. 